The invention relates to the field of record carrier reading and writing control.
The apparatus defined above may be for reading an optical disc, such as the CD or DVD, or may be for reading and writing a writable record carrier. In such an apparatus a reading head generates a read signal from a track on the record carrier, whilst the head is moving relative to the track with a specific velocity. The record carrier is designed to be rotated during reproduction with a nominal rate inversely proportional to the radial position of the read head, so that an information signal with a substantially constant bitrate can be recovered from a record carrier recorded with a constant density. When information is to be retrieved from different parts of the disc the reading head is moved radially at high speed, called a jump, and the rotation rate has to be changed correspondingly to achieve the specific velocity. The changing of the rotation rate causes delays and requires a strong motor.
The above citations are hereby incorporated herein in whole by reference.
The invention aims at providing a reproducing apparatus which is more versatile and is capable of reading information from the record carrier at various speeds.
In accordance with the invention, the apparatus for reproducing information from a record carrier includes
reading apparatus for reading marks from the track, the reading apparatus including a read head for generating a read signal from the track,
A/D conversion apparatus for A/D converting the read signal with a specific clock frequency, in response to a first control signal,
bit detector apparatus having an input coupled to the A/D conversion apparatus, which bit detector apparatus is adapted to retrieve a sequence of bits from the signal applied to its input, with a specific bit frequency, and apparatus for deriving the specific bit frequency,
output apparatus for supplying the sequence of bits,
deriving apparatus for deriving the first control signal, and is characterized in that the deriving apparatus includes apparatus means for deriving from the read signal a second control signal which is indicative of the relative velocity of the read head with reference to the marks, and apparatus for outputting the first control signal in dependence of the second control signal.
The invention has the effect that the A/D converter and further digital elements, such as an equalizer and bit detector apparatus, operate with a substantially fixed ratio between the specific clock frequency and the bit frequency. It is advantageous, that the first control signal is derived from a speed dependent parameter of the read signal itself, as the parameter in the read signal corresponds to the actual speed of the read head with respect to the recorded marks. This results in a reliable recovery of the bit frequency over a wide range of speeds, allowing a fast start of the data recovery following a jump.
In addition the invention is advantageous in that the rotation rate may be adapted more slowly, which results is less strict requirements on the drive motor and the power consumption due to accelerating and braking the disc. When jumps are performed with short intervals the apparatus operates effectively with a substantially constant rate of rotation (Constant Angular Velocity, CAV), whereas when relatively large amounts of date are read the apparatus operates with a substantially constant linear velocity (CLV) of the head relative to the track, because the speed of rotation is slowly changed according to the CLV speed range, i.e. inversely proportional to the radial position of the read head. This has the advantage, that the apparatus adapts itself to the type of application, i.e. having a short access time for applications with many random jumps and having a constant specific bit frequency when reproducing large amounts of information.
A preferred embodiment is characterized in that the deriving apparatus further includes apparatus for deriving a third control signal which has a relationship with the bit frequency, and in that the apparatus for outputting the first control signal are arranged for outputting the first control signal in dependence of the second control signal and the third control signal. This results in that the ratio is controlled to be substantially a value N. This is provided by two control loops, one coarse control loop and a fine control loop. The coarse control loop generates the second control signal and controls the specific clock frequency in such a way that the A/D converter supplies a substantially fixed number of N samples per bit to be detected, irrespective of the relative velocity of the read head with reference to the track. The fine control loop generates the third control signal and controls the specific clock frequency so as to correct for the deviation of the specific clock frequency, expressed in number of samples per bit, from the value of N samples per bit.
As a result, although the apparatus for deriving the specific bit frequency may have a small capture range, the apparatus will very quickly control towards the N samples per bit situation.